Bending toward the light: from Darwin to modern ag

Research in the model plant Arabidopsis thaliana now shows that the hormones that cause phototropism are distributed from the tip of the plant rather than where the plant's stem bends as had been thought.

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A Purdue University-led study may change how scientists think about how some plants bend toward light.

Angus Murphy, a professor of horticulture, said the process, called phototropism, is well-documented in grasses, but has been difficult to resolve in dicots, a large group of flowering plants that includes many agricultural crops. Research in the model plant Arabidopsis thaliana now shows that the hormones that cause phototropism are distributed from the tip of the plant rather than where the plant's stem bends as had been thought.

Charles Darwin and his son, Francis, described phototropic bending in the late 19th century based on experiments in which they were able to block light from reaching the tips of plant shoots and keep the plants from bending toward the light. Their work led to the discovery of auxin, a plant hormone that controls growth functions.

Murphy said the mechanisms that caused phototropism in dicots have been difficult to understand, however. For the past decade, it had been thought that auxin was transported down the stem of dicot plants and then moved laterally to epidermal areas to cause bending. Murphy's team, which included collaborators John Christie of the University of Glasgow and Wendy Peer, an assistant professor of horticulture at Purdue, found that the auxin instead is redistributed laterally at the plant tip and then moves down to the bending point, as is the case in grasses.

"We were looking in the wrong place at the right time," said Murphy, whose findings were reported Tuesday (June 7) in the journal PLoS Biology.

Observing the phenomenon in dicots has been difficult, Murphy said, because testing has to be done on plants grown in the dark. Once those plants receive blue light, which regulates growth, other photomorphogenic processes, such as the uncurling of the top of the shoot and leaf production, begin and make the chemical reactions causing phototropism difficult to observe.

Murphy's team found a simple solution. The plants were grown in the dark, exposed to light to allow photomorphogenic processes occur and then placed back in the dark.

"It basically resets the phototropic response," Murphy said.

Once exposed to light again, the phototropic response could be observed and measured with sensors without the other reactions occurring simultaneously.

"Phototropism is something that everyone who grows a plant in their window knows about," Murphy said. "However, it is also critical to the survival of crop plants, especially at the seedling stage."

The paper in PLoS Biology also showed that when the plant is exposed to blue light, a receprot protein called photropin1 blocks another protein, ABCB19, which is essential for auxin to transport from tip down the plant's stem. This creates a pool of auxin at the tip, which can then be distributed laterally before moving down the stem to the bending point.

"We now know what occurs in the first and last stages of phototropism, but still need to understand how lateral auxin distribution actually occurs," Murphy said. "As part of this work, we tested mutations in every known auxin transporter gene and all of them showed phototropic bending."

The National Science Foundation, U.S. Department of Energy, U.K. Biotechnology and Biological Sciences Research Council, and U.K. Royal Society funded the research.